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Old 03-21-2016, 02:47 PM   #61
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Originally Posted by makobuilders View Post
It is 4.9 liters, 300 cubic inches, 180 hp max rating.
That works out to 0.6 HP per CID.
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Old 03-21-2016, 07:03 PM   #62
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So, I don't get this angst about not working a Diesel engine. Truck drivers often sit in truck stops 12 to 48 hours idling their engines just to have AC. In my own trucking company our engines almost always reached 1 million miles, sometimes more. And think of the cases when the same drivers idle their trucks in the winter for heat.
In almost all of America, this isn't done anymore, FYI.

But yes, "under loading" is way overplayed and worried about.
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Old 03-21-2016, 07:52 PM   #63
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The reason truckers get away w idleing for hours is because they drag 30 tp 40 tons of truck all over the place including countless hills, stops and mountian passes. Peddle to the metal very very often as in 100% load. So different than boats any comparisson is mostly meaningless.
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Old 03-21-2016, 09:06 PM   #64
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In almost all of America, this isn't done anymore, FYI.

But yes, "under loading" is way overplayed and worried about.
This is a bit of quibbling. In much of America this is still done. It was done much more a few years go. I know. I owned a trucking company. Still, no harm to engines. I think my observation stands.
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Old 03-21-2016, 10:06 PM   #65
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That works out to 0.6 HP per CID.
Oops, I inverted the ratio. I think I need math lessons from my kids.

So at cruise speed it works out to 300 cubic inches /140 hp = 2+ ci/hp. At full load 180hp it works out to 1.7 ci/hp.

The interesting thing is that the good ole 6-71 is slightly less loaded at 2.3 ci/hp. However, this little non-technical number I doubt is all that critical. A modern engine with modern metallurgy has got to have huge improvements over the old iron.
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Old 03-22-2016, 08:34 AM   #66
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The reason truckers get away w idleing for hours is because they drag 30 tp 40 tons of truck all over the place including countless hills, stops and mountian passes. Peddle to the metal very very often as in 100% load. So different than boats any comparisson is mostly meaningless.
You beat me to it. My discussion is for chronically, rather than intermittently, under loaded engines. In fact, over the road trucks, along with commercial and fishing vessels, are a great example of extreme longevity when routinely heavily loaded. I've encountered fishing vessel gensets with 15,000+ hrs on the clock, and they run 24/7, operating refrigeration plants at 75%+ load, never having been disassembled. That's simply not a coincidence.

The 2250 was a mathematical error, rather than a typo, guilty as charged, I neglected to deduct the very low load idle speed from the 75% calculation.

For the naysayers, there's no hype or hidden agenda here, rest assured, it's the result of having worked on and evaluated many chronically lightly loaded engines, mostly trawlers, gensets and sailing vessels, having removed scores of sooty and seized turbos (and having had many long discussions with engine manufacturers, often at their production facilities on this subject, including Cummins, Lugger, and Yanmar to name a few)

I visited the Cummins re-manufacturing facility a few years ago where hundreds of engines are rebuilt every day. The production supervisor I spoke with, when I asked about the different engine uses, propulsion vs. genset and the effect that had on their inner workings, said it was easy to see which engines had idled at low load, often over-sized improperly loaded gensets, they were loaded with soot on valves, rings and turbos. The bulk of the engines they were rebuilding were from over the road trucks, for the most part they were filthy on the outside, but surprisingly clean on the inside, just worn out.

If you discount this issue then simply continue doing what you've been doing. Otherwise, periodically load up your engine the way it was designed to be used.
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Old 03-22-2016, 08:42 AM   #67
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So, according to manufacturers, the engines should be loaded for how long and at what frequency? I am thinking about the case of a 40K pound semi displacement hull running at 1600-1800 RPMs - at hull speed - (45-foot boat) using two 330 HP Cummings 6BTA5.9 engines. Top RPM is 2800. If I have correctly followed all the nuances from this discussion, I need only kick it up to 2200-2300 for X period of time for every Y hours operated at hull speed. If this is correct, what are X and Y, according to Cummings?

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Old 03-22-2016, 08:51 AM   #68
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If I have correctly followed all the nuances from this discussion, I need only kick it up to 2200-2300 for X period of time for every Y hours operated at hull speed. If this is correct, what are X and Y, according to Cummings? Thanks, Gordon
Suggest you get the correct answer from Tony Athen's who is located at boatdiesel.com
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Old 03-22-2016, 08:57 AM   #69
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Sunchaser,

I am looking at a data sheet from Tony's site now. I have now found one for the 330 HP engine, but have found one for the 315 hp variety. 75% of 315 HP is 236 HP, which this engine produces at a bit over 1800 rpm. if 1800 rpm consumes 5.2 gal/hour, then I am looking at consuming about 6 gal/hour for a short time, the Y variable in the equation mentioned above. And, it would seem, tracking engine temps would be the critical indicator of the amount of work produced by the engine.

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Old 03-22-2016, 09:01 AM   #70
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This is a bit of quibbling. In much of America this is still done. It was done much more a few years go. I know. I owned a trucking company. Still, no harm to engines. I think my observation stands.
OK, point taken. But how long were the trucks idling, and without any load at all on the engine? Then what happened.... they went out under load for several hours. Not dissimilar to running our Detroits at low RPM all day, then running them up to 80-90% WOT (under load) for awhile.
On the other hand, I think we are heatedly agreeing with each other that the under loading issue is way over played.
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Old 03-22-2016, 10:18 AM   #71
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Suggest you get the correct answer from Tony Athen's who is located at boatdiesel.com
Why wouldn't you suggest he get the correct answer from the engine manufacturer? Nothing at all against Tony Athen's, but I would think Cummins would have the knowledge, especially if he provided them with a serial number of the engine.

That comes back to the general load question and with all the misinformation circulating we simply go by the guidelines from the manufacturer, often found in the owners manual.
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Old 03-22-2016, 10:25 AM   #72
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BandB,

I will definitely be reading the owners manual closely once I close on the boat and have access. It does seem to me that most info should be there.

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Old 03-22-2016, 10:27 AM   #73
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To address the concept of % loading of a main propulsion engine I present the attached diagram for my main engine in my pure displacement hull.

The black curve is the rated horsepower of my engine versus engine rpms.

The red curve is the propeller curve for my boat calculated from the hull form parameters (disp, LWL, BWL, prismatic coefficient, canoe body draft, wetted surface area, among others) and the wind resistance, the propeller details (number of blades, blade form, disc area ratio, diameter, pitch) and the transmission gear ratio. I also included parameters for frictional losses in bearings and additional loading from alternators. This calculated curve has been verified by comparing calculated speed versus rpms with measured speed versus rpms. The match gave a 3% rms error. Note that this is the flat water condition. Note also that my engine is slightly under propped in flat water. That was intentional so that I would be closer to having the correct prop in a normal sea state.

The green curve is the percent loading of the engine calculated by dividing the propeller curve horsepower by the rated horse power at each rpm.

For my engine, which is rated at 3,200 rpms, I achieve 75% loading at 2,900 rpms (not 2,400 rpms which is 75% of rated rpms). I normally run at 2,000 rpms which gives 7 knots at approximately 30% loading.

While this curve is not exactly applicable to other boats, its general form provides a good approximation for displacement and semi-displacement hull forms operating at pure displacement speeds.

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Old 03-22-2016, 11:40 AM   #74
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Why wouldn't you suggest he get the correct answer from the engine manufacturer?
Manufacturer's are constrained by sales/marketing considerations when they address questions such as this. For example Yanmar says that you can run any of their engines continuously at 200 rpm off of rated wot rpm. That is pretty typical of all recreational marine engine manufacturers. To say anything less would put them at a competitive disadvantage. And they are very unlikely to have to pay a warranty claim if that advice is followed because that warranty is limited in time and hours.

But professionals who really know marine diesels, like Tony Athens have a different view. I would listen to Tony over any marine engine manufacturer.

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Old 03-22-2016, 01:13 PM   #75
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TDunn,
What is your power rated rpm?

My boat is the same as yours except for speed and engine rpm. Willy has a Mitsu rated at 3000rpm. It looks like your Yanmar is rated higher and/but if I was to compensate for the rpm my curves should be the same as yours.
Also since you're slightly underpropped (where I'd like to be) A slight adjustment should be made for that ... like perhaps calling 3000rpm 50% of load (or 2950). If your rated rpm is 3800 my figures should match yours.

David,
If Tony Athens has a "view" and not facts then what he says is more like a religion. What you believe rather than what you know.

Steve D says he's seen hundreds of engines that have suffered from under loading and others like Tony Athens say they have never heard of an engine dying from being under loaded. I personally think there are a lot of engines in trawlers out there that are hard starters, smokers w stuck rings and other ailments Steve D talked about in the article in the beginning of this thread. The engines in old trawlers are a bit like old people in senior centers. Still chugging along but far from the way they should be.

This loading issue is a personal thing whereas I gain nothing from a change or no change in how anyone treats their engine on this forum. We all individually make our choices and ..... suffer all the consequences .. whatever they are. My theory is that we all know running real slow isn't best for engines but feel there's somebody out there that will bless it so we can feel good about our misbehaving as engine operators. People will do all kinds of things w support from others that they wouldn't do on their own.

And speaking of "views" here's mine. I don't know Tony Athens at all but I'd follow an engine manufacturer before a web guru anytime. Tony has nothing to loose ... the manufacturer has.
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Old 03-22-2016, 01:27 PM   #76
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Eric, my engine is a Volvo-Penta D2-40 rated for 39.6 hp at 3,200 rpms. Note that my boat is slightly under propped for flat water and slightly over propped for rough water. That is by design so that I am essentially perfectly propped in normal slight chop. I am turning a 20" diameter by 13.35" pitch three blade prop with a disc area ratio of 0.55. Basically, you can simply move the prop curve up or down as needed since the shape doesn't change much if any.
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Old 03-22-2016, 02:32 PM   #77
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I don't know Tony Athens at all but I'd follow an engine manufacturer before a web guru anytime. Tony has nothing to loose ... the manufacturer has.
Eric, you may want to acquaint yourself with Tony Athens and the boatdiesel website. That site is well recognized as the go to Cummins Q and A site for marine diesels (other brands as well). Gordon J's question on Cummins 330s is right up boatdiesel's alley.

The company Tony works for is Seaboard Marine. Look them up. As best I understand they are the largest Cummins Marine dealer in the US if not the world. I'm sure I'll get corrected on this if I misspoke.
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Old 03-22-2016, 03:09 PM   #78
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I have no axe to grind in this discussion since I start out knowing nothing. Eric suggested looking at what the manufacturer suggests. So....

The boat that I have an offer on, and am having surveyed next week has a Cummins QSB5.9-380 HO engine (QSB 5.9L 380 hp High Output). According to the Cummins performance curves the max rated RPM is 3000 which produces 375 bhp. Cummins says that their HO engines are rated for pleasure boat use only and not intended for any commercial application that operate 500 hrs per year or less. The HO engines are intended for variable load applications where full power is limited to 1 hour out of every 8 hours of operation. Furthermore reduced power (ie not full power) must be at or below 300rpm less than max rated rpm.

So what does that mean for this engine? As I read it, it means that Cummins says the engine was designed to be run at rpms between 2700 and 3000 no more than 12.5% of the time. At 2700 rpm this engine will produce 260 hp or just under 70% of its rated horsepower.

So, what does this tell me (someone who knows nothing)? It tells me that in this case the engine manufacture says that I shouldn't run the engine at anything more than 70% load for more than 12.5% of its operational time. They also say that the rest of the time, it should be operated at below 70% load. In essence, Cummins is warning me that engine life is risked if operated at too high a load, but they don't have any warning about operating at too low a load.

Now, maybe I am missing some information from Cummins, but that is what it says on their spec sheets for that particular engine.
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Old 03-22-2016, 03:37 PM   #79
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I have no axe to grind in this discussion since I start out knowing nothing. Eric suggested looking at what the manufacturer suggests. So....

The boat that I have an offer on, and am having surveyed next week has a Cummins QSB5.9-380 HO engine (QSB 5.9L 380 hp High Output). According to the Cummins performance curves the max rated RPM is 3000 which produces 375 bhp. Cummins says that their HO engines are rated for pleasure boat use only and not intended for any commercial application that operate 500 hrs per year or less. The HO engines are intended for variable load applications where full power is limited to 1 hour out of every 8 hours of operation. Furthermore reduced power (ie not full power) must be at or below 300rpm less than max rated rpm.

So what does that mean for this engine? As I read it, it means that Cummins says the engine was designed to be run at rpms between 2700 and 3000 no more than 12.5% of the time. At 2700 rpm this engine will produce 260 hp or just under 70% of its rated horsepower.

So, what does this tell me (someone who knows nothing)? It tells me that in this case the engine manufacture says that I shouldn't run the engine at anything more than 70% load for more than 12.5% of its operational time. They also say that the rest of the time, it should be operated at below 70% load. In essence, Cummins is warning me that engine life is risked if operated at too high a load, but they don't have any warning about operating at too low a load.

Now, maybe I am missing some information from Cummins, but that is what it says on their spec sheets for that particular engine.
That's not dissimilar from what I'm use to seeing. Typically with that they'll have notes on warming it up and getting it up to temperature as well as some shut down notes. The other thing I'm use to seeing with that is a warning if you run at low load an extended amount of time to then run it up to a higher load briefly.

I think you found excellent information. People like Tony Athens might elaborate on it. However, regardless of what anyone says, I would keep in mind the limit on the upper range. The key thing is to realize it is a high output engine and it is not designed to be run at top speed on a continuous basis.

It's much like our Yanmar. We have a 180 hp Yanmar in a rib. It only ways 2400 lbs including the engine. The engine started as a 150 hp engine and is clearly set up for speed and a light load. It has very similar load recommendations as what you're describing.
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Old 03-22-2016, 04:09 PM   #80
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I just posted the following to boat diesel. I'll share the response from Tony, when and if I get it.

"Lengthy discussion on the trawler forum about how diesel engines should be run. Ideas range from needing to periodically load an engine to 75%or more for a brief interval and others say running it nice and easy does no harm. My question is specifically about the 6BTA 330 HP model, pushing a 40K pound semi-displacement hull. My engines reach full RPMS, in fact just a bit more than 2800 at WOT. Props are 24 inch with a 27 pitch. I would love to know whether running this boat at hull speed (7.5 -8 knots) all day is going to kill the engines, or do I need to periodically run them harder. If so, how often, how long and at what RPM.
On the survey last week, 1300 knots gave me 7 knots on relatively flat water, so I am guessing hull speed would come around 1450 or so. These engines are on a trawler, that I want to run as economically as possible, but engine longevity is of course important."
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